1. Field of the Invention
The present invention relates generally to a bidirectional add/drop optical amplifier used in the network node of wavelength division multiplexing (WDM) optical communications. More particularly, it relates to a bidirectional add/drop optical amplifier with an all-optical gain controlling function, which fixes the gain of the optical amplifier by lasing amplified spontaneous emission (ASE) lights through an optical feedback loop.
2. Description of the Related Art
Generally in the network node of WDM optical communications, the add/drop of optical signals are frequently performed to solve network problems or to meet the user""s requests. Due to the cross gain modulation property of the optical amplifier, this add/drop of optical signals changes the power of the surviving channels, which was not added/dropped. In order to suppress the unexpected power change for the surviving channels, several automatic gain control methods such as an all-optical gain control system, a pump control system, and a link control system have been proposed. Among these, the all-optical gain control method fixes the gain of an optical amplifier by lasing the ASE light through an optical feedback loop. And this method gives the advantage of controlling a gain relatively easy by utilizing a pair of fiber Bragg gratings (FBGs) or by using two optical couplers and one optical band pass filter (OBPF).
Furthermore, recently some studies on a self-healing network of WDM optical communication, using a bidirectional add/drop optical amplifier with a pair of optical fibers, are undertaking. The Korean Patent Application 1998-63, applied by the present applicants on Jan. 5, 1998, relates to a bidirectional add/drop optical amplifier realized by a pair of optical fibers, used in a self-healing network of a WDM optical communication. This self-healing network of a bidirectional WDM optical communication has the advantage of reducing the numbers of optical fibers and other elements by half compared with the conventional network of the bidirectional optical communication with two pairs of optical fibers.
In order to apply a bidirectional add/drop amplifier to the actual network node of a WDM optical communication, the bidirectional add/drop amplifier should be equipped with an all-optical gain controlling function.
Therefore, the present invention is contrived in order to solve the above-mentioned problem of existing technology. It is an object of the present invention to provide a bidirectional add/drop optical amplifier equipped with an all-optical gain controlling function by applying a pair of FBGs to the bidirectional add/drop optical amplifier realized by two pieces of optical fibers.
The bidirectional add/drop optical amplifier with an all-optical gain controlling function according to the present invention among the bidirectional add/drop optical amplifiers, which add/drop and amplify bidirectionally propagating WDM optical signals through optical fibers for signal transmission, comprises five parts. (1) One arrayed waveguide grating (AWG) multiplexing/de-multiplexing the bidirectionally propagating optical signals simultaneously and adding/dropping the WDM optical signals. (2) Two bidirectional amplifiers, located at the both sides of the AWG, amplifying the bidirectionally propagating optical signals, and emitting ASE lights with the amplified optical signals. (3) Two OBPFs, connected at the both sides of the AWG, filtering the Rayleigh back scattered signals of the WDM optical signals outputted from the AWG after the add/drop process. (4) Two 3-port circulators, connected to the AWG, the EDFA, and the OBPF, transmitting the amplified signals of the bidirectional amplifier to the AWG, and passing the filtered signals of the OBPF to the bidirectional amplifier. (5) Two power control means, connected between the bidirectional amplifier and the signal transmitting optical fiber at the both sides of the AWG, forming an lasing cavity with the AWG and the OBPF using the ASE lights from the bidirectional amplifier, and constraining the bidirectional amplifier""s output power change due to the adding/dropping the bidirectional signals.
In a preferred embodiment, two power control means comprise two FBGs whose center wavelengths, separated by one FSR of the AWG, are equal to those of the filtered ASE lights at the multiplexing channel of the AWG.